In recent years, a battery pack using a lithium ion battery is installed in a mobile device, such as a digital camera. Generally, it is known that detecting directly a residual quantity of a lithium ion battery at a specific voltage is difficult. For this reason, a method of detecting a battery residual quantity is used. In this method, a microcomputer or the like is used to detect a charging/discharge current of a lithium ion battery and to compute a sum of the detected charging/discharge currents. The battery residual quantity is detected by the result of the computation.
A certain amount of current is consumed in the lithium ion battery even when the lithium ion battery is detached from the mobile device. A fuel gauge IC (a semiconductor integrated circuit) to detect the battery residual quantity of the lithium ion battery and compute the sum of the charging/discharging currents is installed on a printed circuit board together with a protection IC having a regulator function. The printed circuit board and the lithium ion battery are accommodated in a housing, and the fuel gauge IC is offered as a battery pack including the housing.
FIG. 4 is a perspective view showing the appearance of a battery pack. FIG. 5 is a top view of a circuit part of a battery pack according to the related art. As shown in FIG. 4, a printed circuit board 2 is fixed to a top surface of a lithium ion battery 1 having a rectangular shape. As shown in FIG. 5, a fuel gauge IC 3 to detect a battery residual quantity, and a protection IC 4 having a regulator function are fixed to a central portion of the top surface of the printed circuit board 2. A positive power-supply terminal 5, a negative power-supply terminal 6, and a communication terminal 7 are disposed at end portions of the top surface of the printed circuit board 2.
The power-supply terminals 5 and 6 are connected to the positive and negative electrodes of the lithium ion battery 1 via the through holes of the printed circuit board 2. These power-supply terminals 5 and 6 are connected to the positive and negative power-supply terminals of a mobile device (not illustrated). The communication terminal 7 is connected to the communication terminal of the mobile device, and information, including a battery residual quantity and other information, is transmitted and received between the fuel gauge IC 3 and the mobile device.
Patent Document 1 listed below discloses a circuit module in which a protection circuit to protect a lithium ion battery and a charging/discharging circuit to control the charging and discharging of the lithium ion battery are provided.                Patent Document 1: Japanese Laid-Open Patent Publication No. 2008-103219        